Apparatus and method for use in global position measurements

ABSTRACT

A receiver receiving global positioning data from one or more satellites above the Earth&#39;s surface detects a change in at least one parameter associated with the receiver and determines if the change is to be treated as erroneous. As a result of determining if the change in the at least one parameter is to be treated as erroneous, a further action may be performed. Determining if the change is to be treated as erroneous may include, for example, detecting changes in more than one parameter and determining if the changes are coincident. Detecting the change may also enable the receiver to predict the presence and magnitude of multipath components of signals, predict changes in an environment local to the receiver, predict large errors in position estimates determined by the receiver and modify an acquisition and tracking strategy used by the receiver.

INCORPORATION BY REFERENCE TO RELATED APPLICATIONS

Any and all priority claims identified in the Application Data Sheet, orany correction thereto, are hereby incorporated by reference under 37CFR 1.57.

BACKGROUND

The present invention relates to apparatus and methods for measuringglobal position.

The basic functionality of a Global Positioning System (GPS) receiver isdetermining its position by computing time delays between transmissionand reception of signals transmitted from a network of GPS satellitesabove the earth's surface, which are received by the receiver on or nearthe surface of the earth. The GPS satellites transmit to the receiverabsolute time information associated with the satellite signal. Arespective time delay resulting from signal transmission from each ofthe respective satellites to the receiver is multiplied by the speed oflight to determine the distance from the receiver to each of therespective satellites from which data is received. This distance isknown as the pseudorange. If fewer than three satellites are used todetermine a position, the distance may not be precisely determinable dueto an offset between an oscillator in the receiver generating a clocksignal for the receiver and the timing signal to which the satellitesare synchronized. The GPS satellites also transmit to the receiverssatellite-positioning data, generally known as ephemeris data.

The timing signal from each satellite includes a time tag that is usedby the receiver to determine when each received signal was transmittedby each respective satellite. By knowing the exact time of transmissionof each of the signals, the receiver uses the ephemeris data tocalculate where each satellite was when it transmitted a signal. Thereceiver then combines the knowledge of respective satellite positionswith the computed distances to the satellites to determine thereceiver's position.

Position calculations generated from satellite signals may usepseudorange measurements, ephemeris data, absolute time of transmissionand/or differences in arrival time, from four or more satellites todetermine a three dimensional position estimate of the GPS receiver'slocation, which includes latitude, longitude and altitude. Measurementinformation from three satellites is needed to determine a twodimensional position estimate of the GPS receiver's location, whichincludes latitude and longitude.

Other Global Navigation Satellite Systems (GNSS) operate using similarprinciples as GPS, as described above.

SUMMARY

When a GNSS receiver enters a heavily multipath area from a signalenvironment in which the receiver is substantially in a direct line ofsight with most of the space vehicles (SVs) it is receiving signalsfrom, signals from SVs at low elevation tend to suffer from multipatheffects, whereas signals from overhead SVs are less affected. Theexpression “multipath” in this context refers to a phenomenon in which atransmitted signal is reflected by intervening objects such asmountains, buildings or other structures one or more times before itreaches the GNSS receiver. The reflection(s) cause the path length ofthe signal to increase in comparison with a direct path, therebyincreasing the measured pseudorange. The reflected signal may interferedestructively with the direct path signal, reducing its strength.

According to a first aspect of the invention, there is provided anapparatus for a receiver, the receiver having circuitry configured toreceive signals from at least one source providing global positioninginformation, the apparatus comprising a parameter change detectorconfigured to detect a change in at least one parameter associated withthe receiver; and a parameter error determiner configured to determineif the change in the at least one parameter is to be treated aserroneous.

In some embodiments, said parameter change detector is configured todetect a sudden change in said at least one parameter.

In some embodiments, said parameter error determiner is configured todetermine if the change is to be treated as erroneous based on at leastone of (1) a magnitude of the change, (2) a rate of change and (3)whether or not the change is sudden.

In some embodiments, wherein said parameter error determiner isconfigured to determine if the change is to be treated as erroneousbased on further information. The further information may include anyoneor more of: a measurement of said parameter at a different time to thedetected change; a value for said parameter; a threshold change in thevalue of said parameter; a measurement of a different parameter atsubstantially the same time as the detected change, or at a differenttime; and information that describes one or more aspects of theenvironment in an area proximate the receiver, when the apparatus is inuse. Particular examples of further information may include anyone ormore of: a position of said receiver determined by at least one of saidapparatus and said receiver; a clock offset between the clock of thereceiver and the clock of the at least one source; velocity of saidreceiver; acceleration of said receiver; geometric dilution of precision(GDOP): a number of sources from which the receiver receives signals; adetermined pseudorange for a signal received by said receiver; thestrength of a signal received by said receiver; a parameter measured byan inertial sensor; a parameter indicative of the presence or absence ofa multi-path component of a signal received by said receiver; and aparameter indicative of an aspect of the environment in an areaproximate the receiver.

In some embodiments, measurement of said different parameter comprises ameasurement of a change in value of said different parameter.

In some embodiments, at least one parameter comprises one or moreparameters received, detected by, or determined by at least one of saidreceiver and said apparatus.

In some embodiments, the apparatus is configured to cause an action ifthe change in the at least one parameter is to be treated as erroneous.For example, if the change in the at least one parameter is to betreated as erroneous, the action may be performed to mitigate errorresulting from the change.

In some embodiments the apparatus is configured to cause at least one ofthe following actions: search for a direct line of sight signal: modifya weighting value of a signal or discard a signal from one or moresources from which the receiver receives a signal; alter a measuredpseudorange; alter a measured pseudorange based on an estimated amountof multi-path; provide an indication of the presence of one or moremulti-path component(s) of signal(s); one or more of predict, estimate,determine and provide an indication of the magnitude of one or moremulti-path component(s) of signal(s); one or more of detect, determineand provide an indication of information on the environment local to thereceiver; correct a determined position which was determined based onthe at least one parameter having the change; provide an indication of apossible position inaccuracy to a user; one or more of determine andprovide an indication of the magnitude of an error in one or moreparameters; one or more of control and modify a strategy for one or moreof acquiring and tracking signal(s) transmitted from one or moresources; stop using or searching for one or more particular sources;prompt a user for one or more of a response and an input; call upon anenvironment information database used to describe at least one aspect ofthe environment in an area proximate to the receiver; limit the use ofat least one parameter during a period when the at least one parameteris outside an acceptable range, the start of the period identified bythe change in the at least one parameter; modify the at least oneparameter having the change; and identify the change in the at least oneparameter and wait for a subsequent position determination beforedeciding whether the change is erroneous.

In some embodiments, the at least one parameter comprises altitude, andsaid apparatus further comprises a detector for detecting changes inclock offset between the clock of the receiver and the clock of the atleast one source, and said parameter error determiner is adapted todetermine if a change in at least one of altitude is to be treated aserroneous based on a detected change in clock offset.

In some embodiments, an apparatus as described herein may be included ina receiver having circuitry configured to receive signals from at leastone source providing global positioning information.

According to a second aspect of the invention, there is provided amethod for use in a receiver, the receiver having circuitry configuredto receive signals from at least one source providing global positioninginformation, the method comprising: detecting a change in at least oneparameter associated with the receiver; and determining if the change inthe at least one parameter is to be treated as erroneous.

In some embodiments, detecting a change in at least one parameterassociated with the receiver comprises detecting a change in two or moreparameters; and determining if the change in the at least one parameteris to be treated as erroneous comprises determining if the change in atleast two of the two or more parameters is coincident in time; and ifthe change in the at least two parameters is coincident in time,treating the at least two parameters as erroneous or another parameteras erroneous.

In some embodiments, the at least one parameter comprises altitude, andsaid method further comprises directing changes in clock offset betweena receiver clock and a clock of at least one source, and said parametererror determiner is adapted to determine if a change in altitude is tobe treated as erroneous based on a detected change in clock offset.

According to a third aspect of the invention, there is provided computerreadable medium having stored thereon program instructions executable bya processor of a receiver, the receiver having circuitry configured toreceive signals from at least one source providing global positioninginformation, for causing the computing device to perform: detecting achange in at: least one parameter associated with the receiver; anddetermining if the change in the at least one parameter is to be treatedas erroneous.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments of the invention will now be described withreference to the attached drawings in which:

FIGS. 1A and 1B are schematic diagrams of a receiver in communicationwith four satellites, which transmit signals used by the receiver indetermining a position of the receiver;

FIG. 2 is a graph that illustrates altitude of a position fix over aparticular test period for a particular type of receiver;

FIG. 3 is a graph that illustrates altitude: of a position fix over aparticular test period for a different type of receiver;

FIG. 4 is a flow chart describing a method of signal processingaccording to an embodiment of the invention; and

FIG. 5 is a block diagram of an example of a receiver configured toimplement embodiments of the invention.

DETAILED DESCRIPTION

A receiver for receiving global positioning information described hereinbelow may be a receiver for use in a Global Navigation Satellite System(GNSS)—Exemplary types of GNSS include Global Positioning system (GPS),Galileo, Global Navigation Satellite System (GLONASS), Wide AreaAugmentation System (WAAS) and European Geostationary Navigation OverlayService (EGNOS). Moreover, the use of global positioning information isnot to be limited to information for only GPS, but is intended to beinformation generated, processed, and/or transmitted by any type of GNSSsignal source, which is used for determining a position estimate.Similarly, a source of a signal, for example, a space vehicle orsatellite, and the signal transmitted by such a source can be a sourceconfigured to one or more of generate, process, and transmit one or moresignals for one or more of these types of GNSS.

One reason that a pseudorange determined for a signal received from asource, i.e. space vehicle (SV), at a low elevation i.e. closer to thehorizon from the perspective of the receiver) tends to suffer frommultipath effects, is that the pseudorange of an SV nearer the horizonis increased due to multipath that occurs between the SV and thereceiver. The pseudorange of an SV directly overhead is not increased bya substantial amount as it does not experience multipath. For example,if the SV is directly above a receiver that is located in a street of acity core in which there are tall buildings on either side of thestreet, the receiver will receive signals from the SV withoutsignificant multipath, as there is a direct line of sight from the SV tothe receiver. However, the SV closer to the horizon may not have adirect line of sight to the receiver and the signal may reflect off ofthe buildings on one or both sides of the street one or more timesbefore reaching the receiver. This will be described in further detailbelow with reference to FIGS. 1A and 1B.

The SV may be, for example, a satellite or some other source enabled toprovide signals to a receiver used in determining a receiver positionestimate.

For a standard GPS position calculation, it is the difference in thetime of arrival of the different satellite signals that is important,rather than the absolute value of the arrival time. This is because thereceiver's local oscillator has an offset with respect to the satellitetiming signal expressed in GPS Time of Week. After an initialsynchronization between the timing signal of the satellites and thereceiver, the receiver clock offset continues to increase over time. Insome situations the receiver clock may be occasionally reset to coincidewith the GPS Time of Week and reduce the receiver clock offset to zeroat that point in time. In alternative GNSS, the GPS Time of the Week isreplaced with a similar type of timing signal used by the source of theglobal position information signal. In resolving the pseudorangemea9urements to derive a receiver position estimate, an estimate of thereceiver local oscillator offset, (clock offset), is determined for thereceiver at the time of the received signal. In the example above, thedetermined pseudorange estimate for the source having the low-elevationis longer than a direct path from the source at the low elevation to thereceiver, whereas the determined pseudorange estimate for the sourcethat is overhead is substantially the direct path length. Whendetermining the receiver position, the receiver may attempt to combinethe overestimated pseudorange from the low-elevation source and thesubstantially accurate overhead source by performing a “best fit” forboth pseudorange values. The practical consequence of this, asdiscovered by the inventors, is that the pseudo range measurementsprovide: a good fit to a position estimate for the receiver at a higheraltitude than the true position. This higher altitude position estimatesatisfies a condition of being moved comparatively closer to an overheadsource than to the low-elevation sources. This will be described belowin further detail with reference to FIGS. 1A and 1B. Thus, thedetermined position estimate, assuming the receiver is recalculating itfrequently (for example, everyone second is common), will undergo asudden change in altitude, if and when an error in the position estimateis made. In some situations, if recalculation is performed lessfrequently, the change may not appear to be a “sudden” change. In someembodiments, a change may be determined by determining a rate of changeof a parameter and comparing that to a threshold value. The extent ofthe change is related to the amount of multipath in the signals and theangle of elevation of the low-elevation SVs, but in some situations thesize of the change is roughly equal co the average amount of multipath.A consequence of this, also discovered by the inventors, is that thecalculated receiver clock offset will undergo a corresponding change.

Any change in the calculated receiver clock offset, depending on thesize of the change, may be an indication of a sudden presence or absenceof error in the position estimate and/or receiver clock offsetdetermination. This change may be caused by the appearance or thedisappearance of multipath signals. When the change in the receiverclock offset coincides with a change in altitude, the cause is likely tobe multipath. If the change in the receiver clock offset coincides withsome other parameter, such as two-dimensional position, received signalstrength or Dilution of Precision (DoP), then situation-specificconclusions may be drawn and the behavior of the receiver may be alteredaccordingly. For example, as a result of error detected in both thealtitude and receiver clock offset, a specific amount of multipath maybe determined in the signals from low-elevation SV Signals. The receivermay act on the conclusions that are drawn by initiating one or more of,for instance: correcting the final position estimate; searching for theweakened, direct signal; and modifying the weighting value of one ormore source measurements. Modifying the weighting value of one or moresource measurements may include reducing the weighting values ofmeasurements from SVs which are determined to have multipath as comparedto the weighting of measurements from SVs which have a substantiallydirect line of sight to the receiver.

An example of determining a position estimate that results in animproper altitude will now be described with reference to FIGS. 1A and1B.

FIG. 1A illustrates a receiver 100 in communication with foursatellites, Space Vehicle 110, Space Vehicle 120 Space Vehicle 130 andSpace Vehicle 140. The receiver 100 is located on a street 150. Thereare buildings 160 and 165 on either side of the street 150. In theillustrated example, signals from satellites 110, 130 and 140 reflectoff one or more of the buildings 160,165 at least once before reachingthe receiver, causing multipath for these signals. In an environmentsubject to multipath signals, a determined pseudorange is longer thanthe actual (i.e. direct) path between the respective satellite and thereceiver, due to the fact that the signal has taken a longer route. Thesignal from satellite 120 is in a direct line of sight with thereceiver, and therefore, there is no multipath for this signal. For astandard GNSS single point solution in three dimensions, four signalmeasurements are used. The receiver measures the difference between theranges from itself to the satellites rather than the absolute distance.It then determines the absolute distances by finding the unique point atwhich the four pseudoranges meet. In resolving the position at whichthese four pseudoranges meet, the receiver determines the timedifference, or offset, between its own local clock and GNSS time used bythe satellites. In some situations, the clock offset can be used todetermine the absolute distance between satellite and receiver, since itis known at the receiver what each satellite position is and when eachsignal was transmitted.

FIG. 1B illustrates a “best fit” result in determining a positionestimate for the receiver 100. In attempting to compensate for themultipath shown in FIG. 1A, the receiver determines a position that is abest fit for the four signals. The actual position of the receiver 100is indicated at Location B and the determined position is indicated atLocation A. A temporal difference between the time it takes for a signalto travel from where the receiver 100 is actually located, Location B,and where the receiver 100 is determined to be located, Location A,results in a additional temporal offset of the receiver clock withregard to that of the GPS Time of Week value to which the satellites aresynchronized. The temporal offset is revealed as a sudden change in thereceiver clock offset approximately equal to the temporal offsetrequired for a signal to travel between Locations A and B, indicated byDistance C, multiplied by the speed of light. In some situations, thissubstantially corresponds to a value of multipath (i.e. the difference,in time or distance, between the direct path and an indirect path due tomultipath reflections) for one or more satellites that do not have adirect line of sight with the receiver 100. Determining the receiverclock offset may be performed by methods known to those skilled in theart.

When a receiver undergoes a genuine change in altitude, for example anincrease in altitude, a similar, but subtly different phenomenon occurs.The pseudoranges from sources overhead appear to shorten compared withsources at a low elevation. Although there will be a change in altitudein the position fix, this will not correspond to a catastrophic changein the receiver clock offset, as in the multipath example above.

In some embodiments of the invention, a sudden change in a parameterassociated with the receiver may be determined by comparing a detectedmagnitude and/or rate of change to an acceptable threshold far themagnitude and/or rate of change.

In a particular example, a detected rate of change may be compared witha threshold, where the threshold is determined from environmentinformation that includes, for example, topographical information. Thetopographical information may, for example, be used to create thresholdsfor acceptable changes in known altitude in a given region proximate tothe receiver. If there are no known topographical features (a hill or amulti-story building) in an area proximate to the receiver that wouldresult in a change in altitude from a previous measurement, andtherefore, a rate of change greater than an acceptable value, a rate ofchange greater than this acceptable value would be an indication of apotential error in the position determined for the receiver if there areknown topographical features, for example buildings, in an areaproximate to the receiver, this also may be used to predict thepossibility of multipath occurring in proximity to the receiver. Suchinformation may be taken into consideration when setting thresholds fordetecting magnitude and/or rate of change. In another embodiment, thethreshold may be set, to an acceptable value as a function of previousmeasurement, for example, the threshold may be set to an “average”change, or multiple thereof, where the “average” change is an average ofthe changes that were determined for a given number of previousdetermined measurements.

FIGS. 2 and 3 are graphical plots of receiver clock offset versusaltitude determined as part of a position estimate for two differenttypes of receiver. The plots correspond to test data in which thereceivers traversed a path known to have multipath. The results in thetests show an erroneous upward leap in the altitude of the position fix.Typically, a sudden change of 200 m to 300 m was observed in the tests.In FIGS. 2 and 3, the altitude is represented on the vertical axis inunits of meters (m) and the receiver clock offset is represented on thehorizontal axis in units of 10-4 seconds (s).

In FIG. 2, the sudden change in altitude is illustrated by a variationranging between approximately 10 m and approximately 340 m in a range ofreceiver clock offset of 5.58×10⁻⁴ s to 5.60×10⁻⁴ s. In FIG. 3, thesudden change in altitude is illustrated by a variation ranging betweenapproximately −40 m and approximately 220 m in a range of receiver clockoffset of 5.57×10⁻⁴ and 5.62×10⁻⁴ s. A discontinuity 200, 300 in thereceiver clock offset appears between approximately 5.59×10⁻⁴ s and5.60×10⁻⁴ s in FIGS. 2 and 3, as a nil change in altitude during thediscontinuous time duration. This is indicative of a sudden change, i.e.an increase, in the receiver clock offset of approximately 1×10⁻⁶ s. Asthe receiver clock offset should increase at a consistent rate, based onthe difference between the clock of the receiver and the time signal ofthe satellites at any given instant in time, the temporal discontinuityis inconsistent with such a consistent change in offset. However, it wasdiscovered by the inventors, that the amount of the temporaldiscontinuity in the receiver clock offset multiplied by the speed oflight, is substantially equal to the value of the sudden change in thedetermined altitude.

For example, based on a change in altitude of approximately 150 m, areceiver clock will appear to have drifted, or be offset, byapproximately half a microsecond (0.5×10⁻⁶ s). It is noted that thiscorresponds to FIG. 2 in which a change in altitude of 300 m correspondsto a receiver clock offset of 1×10⁻⁶ s.

A result of this observation is that changes due to multipath signalscan be detected by monitoring the receiver clock offset and the reportedposition estimates and looking to for sudden changes that occursimultaneously in these two parameters.

A similar phenomenon, but with an opposite result may occur when aposition estimate is made inside a building that has a good view of alow part of the sky and sources that may be located there, but not adirect line of sight with an overhead source. In this case, the positionestimate may give an erroneously low altitude.

It is of interest to note that a similar phenomenon may be observable inmany different types of environments. For instance, when exiting from acovered walkway where the receiver has a good view of low-elevation SVs,but not SVs which are directly overhead. In such a case, the overhead SVsignals suffer from multipath by the time they are received at thereceiver. If for example the receiver has been turned on while under thecovered walkway, the receiver may have an initial estimated positionthat has an erroneous altitude, i.e. being lower by approximately theamount of multipath in the overhead signals. As the receiver exits to anenvironment: with a good view of the sky, the position estimate may inactuality improve, but a jump in ‘the receiver clock offset and altitudewill appear as a sudden change similar to that described in the exampleabove. In such a situation, it is to be determined which of themeasurements, before or after exit from the covered walkway, is believedto be the correct determined position.

In some embodiments, different methods can be used to help determinewhich of these situations is more likely that is, which estimaterepresents the true altitude, the altitude before or after theidentification of the sudden change. One method may include using alight sensor coupled to the receiver to aid in determining whether thereceiver has access to overhead SVs or not. For example, if the lightsensor only measures a small amount of light, this may be an indicationthat the receiver is in an enclosed space without much direct overheadlight and therefore, without direct line of sight access to an overheadSV. However, if the light sensor measures a larger amount of light, thismay be an indication that the receiver is out in the open and therefore,has direct line of sight access to an overhead SV. Other examples ofmethods that may be used to help determine whether the true altitudeestimate is before or after an identified sudden change may involveusing received SV signal strength or a change in received SV signalstrength, map data including geographic or architectural environmentinformation (see applicant's co-pending PCT Application No.PCT/CA2007/001519, published as Publication No. WO/2008/025150, titled“An Apparatus and Method for use in Global Position Measurements”), andusing the presence and/or absence of a multipath component. In someembodiments, determining whether there is most likely an error in theposition estimate of the receiver is a decision that is made in thereceiver.

Accordingly, some embodiments of the invention involve analyzing one ormore parameters measured or calculated by a GNSS receiver, identifying asudden change in the one or more parameters and treating the one or moreparameters at the occurrence of the sudden change as erroneous.

In some embodiments, if the sudden change in the at least one parameteris to be treated as erroneous, steps are then taken to initiate anaction to mitigate error resulting from the sudden change. A primaryerror that may result from the sudden change is an incorrect positiondetermination by the receiver.

An example of a method for determining a sudden change in at least oneparameter will now be described with reference to the flow chart of FIG.4.

A first step 400 of the method involves detecting a change in at leastone parameter associated with the receiver. A second step 410 involvesdetermining if the change in the at least one parameter is to be treatedas erroneous. A third step 420 involves, in a manner responsive todetermining if the change in the at least one parameter is to be treatedas erroneous, performing an action if the change in the at least oneparameter is to be treated as erroneous. Performing an action may, forexample, be an action to mitigate error resulting from the suddenchange. To mitigate error, the receiver may, for instance, performanyone or more of the following (which is not intended to be anexhaustive list of possibilities): altering a position determinationmade by the receiver, altering a parameter used in making a positiondetermination made by the receiver, and indicating a possibility of anerror to a user.

Determining if the change in the at least one parameter is to be treatedas erroneous, as in step 410, may involve one or more of: performingcorrelation of at least one parameter having the change with environmentinformation that describes an aspect of the environment in an areaproximate to ‘the receiver; performing correlation of at least oneparameter having the change with at least one other parameter or withanother value of the same parameter; and determining if the change isphysically possible. For example, in the case of a detected change in aparameter, a comparison may be made: with another parameter to determineif a change in the other parameter coincided with the detected change.If so, this may be used as an indication that one or both parameters, orchanges in the parameters, can be treated as erroneous. This may be usedas an indication that another parameter, for example, a parameterdetermined from the same data that was used to determine the parametersto be treated as erroneous, may be treated as erroneous.

In some embodiments, the method involves detecting a change in aparameter and characterizing that change. Based on the characterizationof that change it can then be determined if the change should be treatedas erroneous.

Performing an action if the change in the at least one parameter is tobe treated as erroneous, as in step 420, may include at least one of thefollowing actions: (1) searching for a direct line of sight Signal basedupon an estimate for an amount of multipath; (2) modifying a weightingvalue of a signal or discarding a signal from one or mare source(s) fromwhich the receiver is receiving a signal; (3) altering a measuredpseudorange by a value corresponding to an estimated amount ofmultipath; (4) correcting a determined position which was determinedbased on the at least one parameter having the change; (5) issuing anindication of a possible position inaccuracy to a user; (6) stop usingor searching for one or more particular sources; (7) prompting a userfor one or more of a response and an input; (8) call upon an environmentinformation database used to describe an aspect of the environment in anarea proximate to the receiver; (9) limiting the use of at least oneparameter during a period when the at least one parameter is outside ofan acceptable range, the start of the period identified by the suddenchange in the at least one parameter; (10) modifying the at least oneparameter having the change; and (11) identifying the change in the atleast one parameter and waiting for a subsequent position determinationbefore deciding whether the change is erroneous.

Although the embodiments of the invention have been discussed abovepertaining co measuring changes in altitude, changes in other parameterscould be measured instead, e.g. lateral position: clock offset; GDOP(Geometric Dilution of Precision); number of satellites from whichsignals are received for a position measurement; pseudoranges; lightintensity in an area proximate to the receiver, if the receiver iscoupled with a light sensor (useful for changing operation when we havegone inside); velocity or acceleration and possibly other inertialsensors.

An example of a receiver according to an embodiment of the inventionwill now be described with reference to FIG. 5. FIG. 5 illustrates areceiver 500 in communication with two satellites, Space Vehicle 540 andSpace Vehicle 550. While only two satellites are shown in communicationwith the receiver, it is to be understood that the receiver may be incontact with one or greater than two satellites at any given point intime.

In the illustrated example, the receiver 500 includes an antenna 510 forreceiving signals from the one or more satellites. Only a single antennais indicated in FIG. 5, but two or more antennas could be used forreceiving signals from one or more satellites. The receiver 500 includesreceiver circuitry 515 and a parameter change and error determiningdevice 518. The parameter change and error determining device 518includes a parameter change detector 520, a parameter error determiner525, and a response controller 530. In the illustrated example, thereceiver 500 includes a data storage medium 533 (e.g. a memory), forstoring information that may be used to determined whether or not avalue of a parameter, or a change in a parameter is to be treated aserroneous. In this embodiment, the data storage medium 533 includes anenvironment information database 535 that is used to define one or moreaspects of the environment in an area proximate to the receiver.However, not all receivers implementing embodiments of the invention maynecessarily include an environment information database.

In addition to the particular components described above as componentsin the receiver which are related to the invention, the receiver isconsidered to have other components related to the operation of thereceiver, for example, transmit circuitry, hardware and/or software foracquiring and tracking satellites, and hardware and/or software fordetermining position estimates based on received satellite information.

In operation, the receiver circuitry 515 is configured to receivesignals from the antenna from ac least one source that is providingglobal positioning information. In some embodiments, the receivedsignals are passed to the parameter change and error determining device518. In some embodiments, the received signals are passed to othercomponents of the receiver which determine one or more parametersassociated with the receiver, such as, for example, hardware and/orsoftware for determining position estimates based on received satelliteinformation.

The parameter change and error determining device 518 is configured toreceive at least one of: at least one parameter measured by ordetermined within the receiver and information regarding at least oneaspect of the environment in an area proximate to the receiver.

In some embodiments, the parameter change detector 520 is configured todetect a sudden change in at least one parameter associated with thereceiver. For example, the parameter change detector 520 evaluatesparameters such as: (1) a position of said receiver determined by atleast one of said apparatus and said receiver; (2) a clock offsetbetween the clock of the receiver and the clock of the at least onesource; (3) velocity of said receiver; (4) acceleration of saidreceiver; (5) geometric dilution of precision (GDOP); (6) a number ofsources from which the receiver receives signals; (7) a determinedpseudorange for a signal received by said receiver; (8) the strength ofa signal received by said receiver or the change in the strength of asignal received by said receiver; (9) a parameter measured by aninertial sensor; (10) a parameter indicative of the presence or absenceof a multi-path component of a signal received by said receiver; (11) aparameter indicative of an aspect of the environment in an areaproximate the receiver; and (12) output from a light sensor coupled tothe receiver. A position of said receiver determined by at least one ofsaid apparatus and said receiver may include one or both of a determinedaltitude value and a determined lateral position value. Parameters frominertial sensors may be used for determining a direction the receiver isheading; and presence/absence of a multipath component to determine ifthe parameters include a sudden change.

In some embodiments, a change in a parameter may be determined byapplying a threshold as described above.

In some embodiments, the parameter error determiner 525 is configured touse information from the parameter change detector 520 to determine ifthe sudden change detected by the parameter change detector 520 is to betreated as erroneous. To determine if a detected change is to be treatedas erroneous, the parameter error determiner 525 may perform correlationof at least one parameter having the sudden change with environmentinformation that describes at least one aspect of the environment in anarea proximate to the receiver, perform correlation of at least oneparameter having the sudden change with at least one other parameter orwith another value of the same parameter, and/or may determine if thesudden change is physically possible. Correlation may be performed withanother value of the same parameter in a situation where a parameterdetermined to have experienced a sudden change is correlated with thesame parameter at a time before the change was determined or at a timesubsequent to the when the change was determined.

The response controller 530 is configured to control a response of thereceiver if the sudden change in the at least one parameter is to betreated as erroneous. The response controller may initiate one or moreone of the following actions: (1) searching for a direct line of sightsignal; (2) modifying (e.g. reduce or increase) a weighting value of asignal or discard a signal from one or more sources from which thereceiver is receives a signal; (3) altering a measured pseudorange by avalue corresponding to an estimated amount of multipath; (4) altering ameasured pseudorange based on an estimated amount of multi-path; (5)providing an indication of the presence of one or more multi-pathcomponent(s) of signal(s); (6) one or more of predicting, estimating,determining and providing an indication of the magnitude of one or moremulti-path component(s) of signal(s); (7) one or more of detecting,determining and providing an indication of information on theenvironment local to the receiver; (8) correcting a determined positionwhich was determined based on the at least one parameter having thechange; (9) providing an indication of a possible position inaccuracy toa user; (10) one or more of determining and providing an indication (ifthe magnitude of an error in one or more parameters; (11) one or more ofcontrolling and modifying a strategy for one or more of acquiring andtracking signal(s) transmitted from one or more sources; (12) stoppingusing or searching for one or more particular sources; (13) prompting auser for one or more of a response and an input; (14) calling upon anenvironment information database used to describe at least one aspect ofthe environment in an area proximate to the receiver; (15) limiting theuse of at least one parameter during a period when the at least oneparameter is outside of an acceptable range, the start of the periodidentified by the change in the at least one parameter; (16) modifyingthe at least one parameter having the change; and (17) identifying thechange in the at least one parameter and waiting for a subsequentposition determination before deciding whether the change is erroneous.

The environment information database 535 may include for examplearchitectural environment information and/or geographical environmentinformation. Information regarding an area proximate to the receiver canbe determined from the environment information database 535. Theenvironment information database 535 may be stored on a computerreadable medium and be accessible by other hardware and/or softwarecomponents of the receiver 500. For example, the parameter errordeterminer 525, may access the environment information database 535 toperform correlation of the sudden change with information about thelocal environment proximate to the receiver. In some embodiments, theenvironment information database 535 is not collocated with the receiver500, but the receiver is configured to access a remote source to obtainenvironment information in an area proximate to the receiver.

In some embodiments, the receiver 500 may have an environmentinformation database 535 collocated with the receiver, but may augmentthe content of the collocated environment information by accessing aremote source of additional environment information for an areaproximate to the receiver.

The parameter change and error determining device may be physicallyimplemented using software, hardware, firmware or any combinationthereof. For example, a hardware implementation may include usingapplication specific integrated circuits (ASIC), field programmable gatearrays (FPGA), other implementations known to those in the art, orcombinations thereof. To implement the functional components insoftware, in some embodiments a microprocessor capable of performingbasic digital signal processing operations is utilized to implement thevarious functionalities of the parameter change and error determiningdevice.

In another embodiment of the invention, a method is provided for usinginformation that indicates a determined change in at least one parameterto predict the presence and magnitude of multipath components of one ormore signals that are received by the receiver. For example, the methodmay involve the receiver detecting a change in at least one parameterassociated with the receiver. This may include any of the parametersdescribed above, for example altitude and receiver time offset. In amanner responsive to the change detected in at least one parameter, thereceiver may use that information to perform one or more of: (1)predicting, (2) estimating, (3) determining and (4) providing anindication of the magnitude of one or more multi-path component(s) ofsignal(s). The change in the at least one parameter and/or otherinformation, such at the determined pseudorange and/or other informationreceived from the satellite transmitting the signal may be used forpredicting, estimating, determining and/or providing an indication ofthe magnitude of one or more multi-path component(s) of a signal.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed is:
 1. A receiver having position detection and errordetermination capabilities, the receiver comprising: an antennaconfigured to receive at least one signal including global positioninginformation from each of at least three sources; receiver circuitryconfigured to receive the global positioning information, to calculatedistances between the receiver and each of the three sources based atleast in part on the global positioning information, and to determine afirst position based at least in part on the calculated distances; and aparameter change and error determining device in a computing deviceincluding computer hardware, the parameter change and error determiningdevice configured to detect changes in the first position over a timeperiod and in a clock offset between a receiver clock and a clockassociated with the at least three sources, to determine whether thedetected changes in the first position are coincident with adiscontinuity in the clock offset, to estimate an amount of multipathwhen the detected changes in the first position are coincident with thediscontinuity in the clock offset, and to alter the calculated distancebetween the receiver and at least one of the three sources based on theestimated amount of the multipath, the receiver circuitry furtherconfigured to determine a second position based at least in part on thealtered distance.
 2. The receiver of claim 1 wherein the distancesbetween the receiver and each of the three sources are pseudoranges. 3.The receiver of claim 1 further comprising an environment informationdatabase including data defining one or more environmental aspects of anarea proximate to the receiver.
 4. The receiver of claim 3 wherein theenvironment information database includes architectural information forthe area proximate to the receiver.
 5. The receiver of claim 3 whereinthe environment information database includes geographical informationfor the area proximate to the receiver.
 6. The receiver of claim 3wherein the parameter change and error determining device is furtherconfigured to correlate the first position with the data from theenvironment information database when the detected changes in the firstposition are coincident with the discontinuity in the clock offset. 7.The receiver of claim 6 wherein the detected changes in the firstposition include a change in altitude.
 8. The receiver of claim 7wherein the parameter change and error determining device is furtherconfigured to determine whether the change in the altitude is physicallypossible.
 9. The receiver of claim 1 wherein the parameter change anderror determining device is further configured to search for a directline of sight signal based on the estimated amount of the multipath whenthe detected changes in the first position are coincident with thediscontinuity in the clock offset.
 10. The receiver of claim 1 whereinthe parameter change and error determining device is further configuredto indicate a presence of one or more multipath signal components whenthe detected changes in the first position are coincident with thediscontinuity in the clock offset.
 11. A method for use in a GlobalNavigation Satellite System receiver, the method comprising: receiving asignal from each of at least three Global Navigation Satellite Systemsources providing global positioning information; calculating distancesbetween the Global Navigation Satellite System receiver and each of thethree Global Navigation Satellite System sources based at least in parton the global positioning information; determining a first positionbased at least in part on the calculated distances; detecting, with aparameter change and error determining device in a computing deviceincluding computer hardware, changes in the first position of the GlobalNavigation Satellite System receiver over a time period and in a clockoffset between a clock associated with the Global Navigation SatelliteSystem receiver and a clock associated with the at least three GlobalNavigation Satellite System sources; determining, with the parameterchange and error determining device, whether the detected changes in thefirst position are coincident with a temporal discontinuity in the clockoffset; estimating, with the parameter change and error determiningdevice, an amount of multipath when the detected changes in the firstposition are coincident with the temporal discontinuity in the clockoffset; altering, with the parameter change and error determiningdevice, the calculated distance between the Global Navigation SatelliteSystem receiver and at least one of the three Global NavigationSatellite System sources based on the estimated amount of the multipath;and determining a second position based at least in part on the altereddistance.
 12. The method of claim 11 wherein the distances between theGlobal Navigation Satellite System receiver and each of the three GlobalNavigation Satellite System sources are pseudoranges.
 13. The method ofclaim 11 further comprising correlating the detected changes in thefirst position with data representing environmental aspects of an areaproximate to the Global Navigation Satellite System receiver when thedetected changes in the first position are coincident with the temporaldiscontinuity in the clock offset.
 14. The method of claim 11 furthercomprising measuring an amount of light incident upon the GlobalNavigation Satellite System receiver and determining whether thedetected changes in the first position are correct based at least inpart on the amount of the light incident upon the Global NavigationSatellite System receiver.
 15. The method of claim 11 wherein thedetected changes in the first position include a change in altitude. 16.The method of claim 15 wherein an amount of the temporal discontinuityin the clock offset multiplied by the speed of light is substantiallyequal to a value of the change in the altitude.
 17. The method of claim11 further comprising searching for a direct line of sight signal whenthe detected changes in the first position are coincident with thetemporal discontinuity in the clock offset.
 18. The method of claim 11further comprising discarding the signal from one of the at least threeGlobal Navigation Satellite System sources when the detected changes inthe first position are coincident with the temporal discontinuity in theclock offset.
 19. The method of claim 11 further comprising providing anindication of one or more multipath signal components when the detectedchanges in the first position are coincident with the temporaldiscontinuity in the clock offset.
 20. The method of claim 11 furthercomprising calling upon an environment information database includingdata describing environmental aspects of an area proximate to the GlobalNavigation Satellite System receiver when the detected changes in thefirst position are coincident with the temporal discontinuity in theclock offset.